The invention relates to a process intended to allow regulation of a line for deinking a paper pulp obtained from recycled or reclaimed paper.
It also relates to the device capable of implementing this process and, more generally, to a device capable of continuously measuring the quantity of particles contained in a liquid, said particles being especially in two different physical states, in particular in the individualized state and in the state bonded to an entity of larger size, such as a fiber for example, these entities being in particular microscopic and being separated beforehand by mechanical means according to dimensional criteria.
The invention relates more particularly to the continuous separation, by screening and washing, of individualized microscopic particles in a suspension from microscopic particles bonded to other more voluminous entities in said suspension, separation being followed by a continuous measurement, by image analysis, on the separated suspension flows of the various fractions of particles.
As already mentioned, the invention is more particularly intended for the papermaking industry, especially for monitoring and regulating the operations of deinking paper pulps coming from reclaimed paper, by providing means for separately measuring the fractions of ink particles "attached to" and "detached from" the papermaking fibers, especially cellulose fibers, this being carried out directly and continuously in the pulp at various points in the deinking process.
Of course, although in the rest of the description the invention is more particularly described with regard to its application to the papermaking industry, it should not be understood to be limited to this single application, and it may also be employed in other fields requiring continuous and separate monitoring of microscopic particles in suspension, according to their nature, associated with dimensional criteria, especially in the field of water treatment and in that of the chemical or agrifood industries.
The recycling of reclaimed paper requires, in particular in the case of printed wastepaper which has to be recycled in the manufacture of white paper, especially of the graphics-paper type, the implementation of various deinking operations aimed at removing the printing inks associated with these materials. Removal of these ink particles necessitates first of all that they be individualized, that is to say correctly separated from the paper printing medium, and in particular that they be detached from the fibers so as to be able to be selectively extracted from the suspension of recycled fibers.
The separate and continuous control of the attached and detached ink particles proves to be very advantageous whenever the ink-detaching operations, especially pulping and hot slushing, and the ink-removal operations, especially washing and flotation, make use of different technologies and different chemical auxiliaries.
Conventionally, the deinking processes include most of the following steps :
making a pulp from wastepaper by pulping, that is to say by mechanical action in the presence of chemicals which promote defibering of the paper and detachment of the ink from the fibers ;
precleaning, by screening on a screen having holes and slits and by cycloning, intended to remove the voluminous and heavy impurities ;
deinking by flotation and/or washing, aimed at removing the ink particles provided that they have been separated from the fibers during pulping ;
fine cleaning by slit screening and cycloning in order to remove small impurities, both heavy and light;
hot treatment aimed, on the one hand, at detaching the ink particles remaining fixed to the fibers and, on the other hand, at dispersing or fragmenting the residual impurities to below the visibility threshold (printing fragments and various impurities, especially those which are heat-fusible); and
bleaching of the pulp, possibly combined with hot dispersion when the bleaching products are introduced upstream of the hot treatment.
Other steps may be added to these operations, in particular when the quality requirements of the deinked pulp are very high in terms of whiteness and cleanness. These steps essentially consist of:
postdeinking by flotation and/or washing, intended to remove inks which have been detached during hot dispersion;
postcleaning by screening through fine slits and/or cycloning, for finishing off the cleaning, especially because of the shape modifications undergone by impurities of the type generally referred to by the term "stickies" (glues and adhesives) during the hot treatment;
final hot treatment for dispersing the residual impurities and inks to below the visibility threshold;
complementary bleaching of the type which is in principle different, that is to say reducing when the first bleaching is oxidizing, and vice versa.
The size of the ink particles in recycled pulps is typically of the order of a few micrometers. The size distribution of these particles can vary considerably depending on the nature of the inks, that is to say can include sizes below one micrometer, in the case of certain inks of the water-soluble type (flexographic printing, etc.), or in the visible range even up to several hundreds of micrometers, in the case of certain inks of the heat-fusible type (laser inks, etc.).